Riddled with faults, Greece can only quake
Greece is one of the best natural laboratories for seismologists wishing to carry out experiments, with few other places on the planet experiencing so much earthquake activity in such a restricted area. An earthquake scoring five on the Richter scale takes place every 18 days in Greece or the greater region, with quakes as strong as – or stronger than – 5.5 Richter taking place every two months. One earthquake of up to 6.3 Richter takes place yearly, while an 8-Richter quake takes place every half century. Under the earth’s surface, two tectonic plates, the African and European, grind against each other squeezing the Aegean Sea and slowly twisting it out of its present shape. At the rate of a centimeter a year, the sea is pushing toward Africa, which is simultaneously pressing against Europe. Plate collision At some point in the future – in a few million years – the Mediterranean will cease to exist and it will be possible to travel overland from Athens to Cairo. But for the present, the powerful earthquakes that often strike the area are the signs of this underground collision. Earthquakes have not only caused villages and cities to disappear but also entire civilizations in the southeastern Mediterranean basin and in the Adriatic, and still pose a threat in the future. A University of Thessaloniki research team has located the faults which have caused the most powerful earthquakes in Greece and the broader region over the past 2,500 years, which, they claim, will be the source of quakes for another 2,000 years. 570 powerful quakes The scientific team of the Aristotle University of Thessaloniki – composed of Vassilis Papazachos, emeritus professor of the Geophysics Laboratory, D. Boudrakis, geology professor, G. Karakaisis, assistant professor of geology, Costas Papazachos, assistant professor of physics-geology, and M. Tranos and A. Savaidis, geology researchers – have ascertained that 570 of the most powerful earthquakes (over 6 Richter) since the fifth century BC until today have been caused by a total of 159 faults. The biggest of all is the Elafonisos fault, southwest of Crete, which is 200 kilometers in length and responsible for the most powerful earthquake in the Mediterranean in AD 365, estimated as 8.2 on the Richter scale, which caused terrible damage as far away as Asia Minor (present-day Turkey) and Africa. Second on the list is the 130-kilometer-long Kastelorizo fault, which caused the 1303 earthquake of 8 Richter that killed 4,000 people. The earthquake map published by Kathimerini will be presented at the second conference on Anti-Seismic Mechanics and Seismology Technology, which began yesterday in Thessaloniki and runs through tomorrow, and will be of great use in both the prevention and prediction of earthquakes. The research team has drawn up a map of the sources of future seismic danger – the faults – which is more specific than mapping general areas prone to earthquakes. Protection measures Now scientists and civil defense officials know where the danger of future large earthquakes lies and can increase anti-seismic measures in that area. Earthquake regulations and safety measures, based on the map, will become more reliable and effective. Besides everything else, it is scientific food for thought, said G. Karakaisis. But how do scientists know when and where large earthquakes struck in antiquity when the recording of earthquakes by instruments only began at the beginning 20th century? Costas Papazachos explained, We know facts about the damage caused by earthquakes which derive from recorded historical testimony; there are some astoundingly detailed descriptions. The biggest earthquakes in Greek history On the night of July 21, AD 365 the greatest part of the Roman world was shaken by a violent and destructive earthquake. The shores of the Mediterranean were left dry by the sudden retreat of the sea. The tides soon returned with the weight of an immense and irresistible deluge, wrote historian Edward Gibbon (1737-1794) on the Mediterranean’s most powerful earthquake, 8.2 on the Richter scale and originating in the Elafonisos fault. In Alexandria, the tsunami lifted the ships anchored in the harbor so high that they were borne over the top of high buildings and walls, and landed in the yards and rooms of houses. The sea then receded, leaving the vessels high and dry. The residents hastened to plunder them and seize the ships’ cargo, but the tsunami surged back and drowned them all, wrote the Byzantine historian Theophanes (ca. 758-818). He went on to relate how sailors in the Adriatic found the retreating waves had left them beached on the seabed, before the waters returned and they floated back up again. Another Byzantine historian, Cedrenus, added that the tidal wave engulfed 5,000 people. On Crete, in Achaia, Boeotia, Epirus and Sicily, many places were lost to tidal waves. Gibbon wrote that huge waves drowned the inhabitants of the coastal areas of Dalmatia, Greece and Egypt. The peaks of Mt. Taygetos were split, the Temple of Zeus at Olympia collapsed, as did buildings, walls, fortifications and columns. Historians also recorded a powerful earthquake that shook Santorini in 1650, which is calculated as measuring 6.8 on the Richter scale. The tremors began in 1649 and were accompanied by an unprecedented rise in temperature. On September 14, fresh tremors began and lasted until the end of the month, reaching their climax on September 27 and 29, when they became more violent, says one account. They were accompanied by a subterranean volcanic eruption with a huge quantity of ash. An island with a crater was formed, vomiting huge, flaming rocks and volcanic ash, which was carried as far as Asia Minor where it even formed a thin layer of ash on tree leaves. Over 200 houses collapsed. Forty villagers and many animals and birds died as a result of the toxic gases while a number of domestic animals lost their sight for over a week. A ship rounding Cape Kolumbo became stuck in floating pumice and its nine crew members were killed. And the tsunami which was formed reached a height of 30 meters on the western coast of Patmos. On Sikinos, the sea retreated 180 meters. The earthquake was felt as far away as Constantinople. Only on December 6 did calm begin to return. The freshly formed volcanic island sank and a reef was formed where it had been at the depth of 18 meters, which was named Kolumbo. Danger zones have changed The new map of areas prone to seismic activity is being drawn up by the scientists of four seismology research units and will be added to anti-seismic regulations in force since the beginning of 2000. Until now, Greece was divided into four seismic danger zones which were established in 1989 and were incorporated into earthquake regulations. However, in the 12 years since 1989, new facts have come to light and more experience accumulated on seismic activity in Greece. Powerful earthquakes, such as that of Kozani and Grevena, or smaller but destructive ones, such as that of Athens in September 1999, overturned previous notions, especially with respect to large metropolitan areas, and have brought new scientific data to light. The new map of danger zones is being drawn up on the basis of this expertise, said the president of the earthquake protection organization OASAP, Vassilis Andrikakis.